skeeve@Skeeves-PowerBook:~ $ perl -e '"<" =~ qr[(??{/^</})]'
Bus error
skeeve@Skeeves-PowerBook:~ $ perl -v
This is perl, v5.8.6 built for darwin-thread-multi-2level
(with 3 registered patches, see perl -V for more detail)
Copyright 1987-2004, Larry Wall
Perl may be copied only under the terms of either the Artistic License+ or the
GNU General Public License, which may be found in the Perl 5 source ki+t.
Complete documentation for Perl, including FAQ lists, should be found +on
this system using `man perl' or `perldoc perl'. If you have access to+ the
Internet, point your browser at http://www.perl.org/, the Perl Home Pa+ge.

You need a re-entrant regexp engine to do what you tried. I don't think even the latest bleads are. Also, your pattern isn't terribly meaningful. Are you *sure* you meant to use the return value of a boolean regexp as another regexp pattern to compile and apply?